1. Field of the Invention
In an active matrix liquid crystal display device, a method of applying a direct voltage and monostabilizing the liquid crystal molecules is shown. In particular, a method of monostabilizing the liquid crystal with a layer structure such as a smectic liquid crystal is shown.
This invention may be applied to a method for monostabilizing a liquid crystal which is bistable such as a ferroelectric liquid crystal.
2. Description of the Related Art
The liquid crystal display device has the advantages of being thin, light weight and low in power consumption. The liquid crystal display device is also used in fields where high speed response of a dynamic level is required such as portable television and wall television. Further, a demand for a large screen display and a projector panel such as a 50 inch rear projector is increasing.
As a liquid crystal orientation mode a TN (twisted nematic) mode of the cell gap which is about 4 to 5 μm is generally used due to the easiness of orientation control. However, the TN mode is slow in the response speed of a halftone display near the white level, and in the case of a high speed dynamic display, the liquid crystal response cannot keep up thereby showing flickers.
A flicker is a phenomenon where in a halftone with slow response speed of the liquid crystal, the liquid crystal response cannot track the greed when the image switches, and a tone that is different to that of a display image appears on the screen and can be seen as a flicker. When an object to be displayed moves at a high speed, such a flicker may be seen in the contour where the previous scene and the scene to which it has been switched greatly change in the display tone.
As the material that changes to a nematic liquid crystal, there is a liquid crystal with spontaneous polarization. With the interaction of spontaneous polarization and an electric field, a high speed response for switching at a micro second level may be performed. A smectic liquid crystal with spontaneous polarization is used in a simple matrix method liquid crystal display device by using a high hysteresis steep threshold characteristic.
Of the liquid crystal with spontaneous polarization, the ferroelectric liquid crystal shows hysteresis in a voltage-transmittivity characteristic by making the cell gap thinner, and it is known to be a bigtable material having a memory property. Conventionally these characteristics were utilized and applied in the simple matrix method liquid crystal display device.
However, in the case of displaying the image in the mode having a memory property, there is a need to display an image in the on and off state of the light. In order to realize the halftone of such as gray scale, there is a need to change the display time and control the gradation.
For such a control, there is required a process of an image signal with a complex software, and a large scale circuit with a complex hardware. Further, for measures against problems of display unique to the time scale such as false contours that may be seen due to blinking and action of the observer, and color breakup, a large number of processes for software and hardware are needed. In this way, aside from the actual panels, even in the peripheral circuits, there is a large burden on cost and design.
Further, even if a liquid crystal which moves at a high speed with the above method is used, for realization of a halftone, there are needed a plurality of sub-fields for control of the light flickering period, so that for display of the image the characteristic was not satisfactorily used.
Therefore by applying an analog value voltage to the liquid crystal that can respond at high speed, there is tested a realization of a displaying method.
As a smectic liquid crystal that can perform analog gradation, recently the development of a polymer stabilized ferroelectric liquid crystal (PS FLC) which has monostabilized the ferroelectric liquid crystal with liquid crystal polymer, and obtained a characteristics without hysteresis is progressing. By injecting the ferroelectric liquid crystal added with a small amount of liquid crystal polymer (2 wt %) to a liquid crystal panel, and applying a direct voltage of about 1 to 15V whilst irradiating ultraviolet rays onto the entire surface of the liquid crystal panel, the polymer stabilized ferroelectric liquid crystal may suppress the hysteresis characteristic therein and obtain an analog gradation.
Note that, polymer stabilization refers to the process of adding light and heat to a liquid crystal and a liquid crystal polymer for polymerization reaction of the liquid crystal polymer. Further, monostabilization is a process for eliminating a bistable state or hysteresis seen in ferroelectric liquid crystal, and obtaining an analog characteristic that fixes transmittivity by an electric field. When a polymer is stabilized whilst applying a single polarity voltage, one of the orientation state of the bistable ferroelectric liquid crystal is strongly stabilized, and is considered that a monostable state is realized (FPD Intelligence 1999. 2, p 78-82).
FIG. 24A is a voltage-transmittivity characteristic of a bistable ferroelectric liquid crystal, and FIG. 24B is a monostable ferroelectric liquid crystal characteristic. The lateral axis shows the voltage and the vertical axis shows the transmittivity. Since the bistable ferroelectric liquid crystal memorizes the arrangement state where the orientation direction of spontaneous polarization formed by application of electric field is matched, the hysteresis is large. A monostabilized ferroelectric liquid crystal is realized with an analog characteristic where the memory property and the hysteresis are eliminated and the transmittivity gradually increases when voltage is applied.
FIG. 23 shows a top view of a simple matrix type liquid crystal display device. According to the simple matrix type liquid crystal display device, the substrate with scanning lines (Y1 to Y8) arranged in a stripe shape in the row direction, and the substrate with signal lines (X1 to X8) arranged in a stripe shape in the column direction are pasted together so that the signal lines and the scanning lines are orthogonal, and is configured by filling liquid crystals in between the substrates.
According to the liquid crystal display device formed of transparent electrodes patterned into 8 columns in the vertical direction as shown in FIG. 23 and transparent electrodes patterned into 8 columns in the lateral direction, the polymer stabilized ferroelectric liquid crystal has attained driving in the field sequential method, and has expectations as an orientation method for high speed response (Semicon-news FORUM 21 preliminary report p 7-13, date of lecture Feb. 24, 2000).
According to a simple matrix liquid crystal display device as shown in FIG. 23, when a monostabilization process is performed, a direct current voltage is applied from a direct current power source in between the rectangularly patterned conductive film, and the orientation axis of the liquid crystal molecules are matched to a one direction of a cone.
However, according to the simple matrix liquid crystal display device, there is a problem that when the number of rows of the scanning line increases, the contrast of display significantly decreases. Recently, in the active matrix liquid crystal display device that may realize a high definition and a high contrast, an analog signal is applied to control an image, and to obtain an image quality with good display characteristics is realized.
However, according to the active matrix liquid crystal display device, the circuit is structured with the alternating current drive originally as the main object, and there was not proposed a method of stabilizing the polymer by a direct current power source or a direct current voltage.
According to the active matrix liquid crystal display device, since the pixel electrode is independent through the pixel TFT, a signal necessary for monostabilization from the outside may not be directly applied to the pixel electrode.
In the conventional active matrix liquid crystal display device, a method of applying a voltage of a single polarity over a long period of time to a liquid crystal with a convenient method and performing a polymer stabilization process is required.
The present invention of the active matrix liquid crystal display device, discloses means for applying an electric field to the mixture of a polymer material added with liquid crystal and polymerization agents, and adding an energy and hardening the polymerization agent by a chemical reaction.
Note that, this energy is an energy to be added for chemical reaction of the polymerization agent. The method of adding energy is light irradiation or adding heat. When adding a light polymerization agent to the polymer material, the polymerization agent may have a chemical reaction by light irradiation. In a material absorbing i line, g line, and h line, the ultraviolet rays are irradiated to the polymerization agent to start the light reaction. When adding the thermal polymerization agent to the polymer material, the chemical reaction of the thermal polymerization agent by heating is started.
First, in explaining about the polymer material added with a polymerization agent, when the added polymerization agent has a light polymerizing property or a heat polymerizing property, the orientation of the liquid crystal is stabilized by a bridging reaction by energy such as light and heat. In this way, a liquid crystal characteristic with a differing characteristic compared to that before applying energy (for example, a threshold characteristic) is obtained. As a polymer material a liquid crystal polymer may be used.
When a liquid crystal polymer is used as a polymer material, a bridging reaction of a liquid crystal polymer occurs by adding energy to the added polymerization agent. The liquid crystals are oriented along the bridged liquid crystal polymer side chain, to obtain a stable orientation. When a bistable liquid crystal orientation is stabilized to a one direction, the threshold characteristics and the like of the liquid crystal changes as compared with when energy is applied.
As the liquid crystal, for example, a smectic liquid crystal as researched in Science University of Tokyo in Yamaguchi, for example, a ferroelectric liquid crystal may be used (Semicon-news FORUM 21, preliminary report, p 7-13, lecture date: Feb. 24, 2000).
In the active matrix liquid crystal display device, there is a limit in the voltage value that may be applied to the liquid crystal layer, but according to the present invention, a large voltage may be applied to the liquid crystal layer of an active matrix liquid crystal display device. Further, in the circuit of an active matrix liquid crystal display device with an alternating current drive as a main object, a direct current voltage may be applied to a liquid crystal.
The present invention is characterized in that as shown in FIG. 10, an electric field is applied to a mixture of a liquid crystal and a polymer material by a transparent conductive film 510 formed on a substrate 508 and a conductive sheet 300 formed on a substrate 400, and simultaneously, energy is added to the mixture of a liquid crystal and a polymer material. If the electric field is applied by using a direct-current power supply, a direct voltage can be applied.
FIG. 10 is a cross sectional view of the pixel portion and the terminal portion of the active matrix liquid crystal display device. With the configuration of FIG. 10, an arbitrary voltage can be applied to the liquid crystal layer. Further, electrodes for applying a voltage to the liquid crystal layer are the transparent conductive film 510 on the opposing electrode and the conductive sheet 300 formed on a back surface of the element substrate. Therefore, a direct voltage can be applied to the liquid crystal by using a conventional liquid crystal display device as it is. With the configuration of FIG. 10, an arbitrary direct voltage can be applied to the liquid crystal layer. However, the voltage is applied to the liquid crystal layer through the element substrate, and thus, a high voltage needs to be applied in order to supply a predetermined voltage to the liquid crystal layer. Therefore, a device for supplying such a predetermined voltage, such as a direct high-voltage power supply is required,
Further, the present invention is characterized in that as shown in FIG. 6, after a conductive film 200 is formed on the element substrate 400, an element is formed. The electric field is applied to the mixture or a liquid crystal and a polymer material by the transparent conductive film 510 formed on the opposing substrate 508 and the conductive film 200. Simultaneously, energy can be applied to the mixture of a liquid crystal and a polymer material. If a direct-current power supply is used, a direct voltage can be used as the voltage applied to the mixture.
FIG. 6 is a cross sectional view of the pixel portion and the terminal portion of the active matrix liquid crystal display device. With the configuration of FIG. 6, an arbitrary voltage can be applied to the liquid crystal layer. Further, since a voltage is applied between the conductive film 200 on the element substrate and the transparent conductive film 510 provided on the opposing substrate through a first interlayer film 457 and a second interlayer film 458, the value of a direct-current power supply for applying a direct voltage is not necessarily so large. On the contrary, one electrode for applying the direct voltage to the liquid crystal layer is the conductive film 200 provided on the element substrate 400. A TFT is formed above the conductive film, and thus, the temperature of the process of forming the TFT is limited by heat resistance of the conductive film.
Further, the present invention is characterized in that in the active matrix liquid crystal display device, as shown in FIG. 2, while a voltage with single polarity is applied to the pixel electrode in continuous frames, the liquid crystal is made to response at a predetermined position, and thereafter, while a voltage applied to the liquid crystal layer by a storage capacitor is retained, energy is applied to the mixture of a liquid crystal and a polymer material. Thus, there is obtained the same effect as that by applying a voltage to the liquid crystal layer by the direct-current power supply.
FIG. 2 shows a timing chart in case of operating a liquid crystal display device with a line sequential driving and an optical response of a liquid crystal layer. In FIG. 2, external signals input to the liquid crystal display device, signals input to a pixel portion of the liquid crystal display device, and voltages applied to pixels of the pixel portion are shown. A gate start pulse 103, and a gate clock pulse 104 as the external signals, a source driver output 112 and a gate pulse 106 as the signals input to the pixel portion, and electric potentials of the pixels connected to gate lines g1 to gn of the pixel portion are shown.
With the configuration of FIG. 2, the liquid crystal layer is made monostable by changing a signal input from the outside in the conventional liquid crystal display device. It is necessary to set the external signal such that a signal output to a source wiring has the same polarity over the plurality of frames. On the contrary, since a direct voltage can be applied to the liquid crystal layer for a long period with the transparent conductive film on the opposing substrate and the pixel electrode of the element substrate, the value of the direct-current power supply for applying the direct voltage is not necessarily so large. If only setting of the external signal is changed, the direct voltage can be applied to the liquid crystal layer in the conventional liquid crystal display device or the driver circuit of the conventional liquid crystal display device.
Further, according to the active matrix liquid crystal display device of the present invention, the plurality of continuous frames as shown in FIG. 1 maintain the gate start pulse 114, to be input to the gate driver from the outside, at a certain level, and are made so that a charge may be supplied to the liquid crystal layer and the storage capacitance at all times. As a result, even if there is a leak of current such as the storage capacitance, the fluctuation of the voltage applied to the liquid crystal layer may be prevented. Then, a feature that the same polarity voltage is applied to the pixel electrode, and energy is added to the mixture of the liquid crystal and the polymer material. As a result, the same effect as the case where voltage is added to the liquid crystal layer by a direct-current power supply is obtained.
FIG. 1 shows a timing chart when the lines are driven subsequently and an optical response of the liquid crystal layer. FIG. 1 shows an external signal to be input to the liquid crystal display device, a signal to be input to the pixel portion of the liquid crystal display device, and a voltage to be input to the pixel of the pixel portion as an external signal. A gate start pulse 114 which is input to the liquid crystal display device as an external signal, a gate pulse 115 to be input to the pixel portion, and the potential of the pixels connected to gate lines g1 to gn differ to FIG. 2.
The configuration of FIG. 1 needs to change the gate start pulse of the external signals compared to FIG. 2. However, similar to the configuration of FIG. 2, a direct current voltage may be applied to the liquid crystal with the configuration of the driver circuit of the conventional liquid crystal display device and the conventional liquid crystal display device.
The advantages and disadvantages of each configuration of this invention are compared, and as a method for applying a direct current voltage to a liquid crystal in an active matrix liquid crystal display device, a more generalized method is a method where the liquid crystal display device has a conventional configuration as shown in FIGS. 1 and 2, and the external signal to be input to the liquid crystal display device is altered.